This invention relates to a novel negative working photoresist composition particularly suitable for use on methyl methacrylate polymer substrates and to a method of forming a photoresist on a methyl methacrylate polymer substrate. Further, this invention also relates to a photolithographic method for forming a black light-absorbing matrix on polymethyl methacrylate rear-projection TV viewing screens.
In general there are two methods known in the art for providing methyl methacrylate polymer substrates such as polymethyl methacrylate with a light sensitive photoresist layer. One of these methods involves the use of a polyvinyl alcohol (PVA) negative acting photoresist composition containing water and/or alcohol solvents such as that described in U.S. Pat. Nos. 3,558,310 and 3,922,395.
More particularly U.S. Pat. No. 3,558,310 shows a photolithographic method of producing a light-absorbing matrix on the glass face plate of a color television picture tube. In the method described, the surface of the face plate, after being cleaned, is coated with a photosensitive layer, the solubility of which is altered by exposure to light, by applying to the surface of a face plate an aqueous solution of polyvinyl alcohol and ammonium dichromate and, after drying the layer, exposing the layer to ultraviolet light rays through a mask in three different positions, each position coinciding with desired phosphor locations. Each exposure causes the exposed areas to harden and become water-insoluble.
However this method does not yield good results when applied to polymethyl methacrylate screens because of the poor ability of the polyvinyl alcohol aqueous solution to wet the surface of the polymethyl methacrylate screen.
The addition of a wetting agent allows a negative-working aqueous polyvinyl alcohol photoresist composition to adequately wet the surface of a polymethyl methacrylate substrate. However, the wetting agent remains in the dried photoresist layer as a result of which the supported water-insoluble unsaturated portions of the exposed areas of the photoresist layer may still remain sufficiently water soluble to be removed during development with water. The addition of the wetting agent to the aqueous polyvinyl alcohol photoresist compositions is therefore of little value when the photoresist compositions are used to provide sharply defined patterns on a methyl methacrylate polymer substrate such as is required, for example, for the formation of a black light-absorbing matrix on polymethyl methacrylate rear-projection TV viewing screens.
Another known process employs an all organic photoresist composition such as Kodak Thin Film Resist (KTFR) in which the solvent is xylene. However, while this organic composition adequately wets the surface of a methyl methacrylate polymer substrate and allows precise patterns to be formed on a methyl methacrylate polymer substrate, the use of a composition employing an organic solvent such as xylene introduces other problems as such a composition has all the disadvantages inherent in the use of organic compositions, such compositions being toxic, flammable or explosive and expensive. Thus when employing such compositions special precautions are required such as the use of expensive exhaust systems, face masks and explosive-proof rooms.
As has already been stated, an additional aspect of the instant invention relates to a method for the formation of a black light-absorbing matrix on a polymethyl methacrylate rear-projection TV viewing screen.
In a commonly employed type of such a viewing screen, one surface is entirely covered with contiguous longitudinal, lenticules each lenticule being about 40 mils in width. The television picture is projected on the lenticular side of the screen and is seen as a "focussed" picture on the viewing side. The "focussed" picture, when examined closer, is shown to consist of a series of narrow bright stripes each opposite the center line of a lenticule alternating with a series of dark, unlit areas.
In order to enhance the picture contrast and reduce reflection of ambient light to the viewer, the unlit areas between the bright stripes are coated with a black light-absorbing material. Such a material, while reducing reflection of the ambient light, does not substantially reduce the brightness of the picture.
In one manufacturing process, the entire viewing side of the screen is grooved with longitudinal grooves positioned so as to coincide with the bright stripes of the "focussed" picture. The raised ridges between the grooves which extend up from the screen are contact printed with a black ink to thereby produce a black matrix covering all unlit areas of the picture.
Although the screens produced by this contact printing method are commercially useful there is room for improvement. The black stripes made by the contact process have ragged or scalloped edges, rather than straight edges because of the lack of precision inherent in the contact process. The "window" stripe width, that is the areas of the picture may vary from one area to another for the same reason. Further, the printing process is not suitable for mass production methods as it is not highly reproducible. A further disadvantage of the contact printing method is the need for the longitudinal grooves in the viewing surface of the screen in order that all of the non-lighted areas of the screen be contacted with ink to produce the black matrix. For these reasons, a photolithographic method as described would be particularly desirable for use in providing a black matrix on a rear-projection TV viewing screen formed of a methyl methacrylate polymer substrate.